A battery is an electrochemical energy storage device. Batteries can be categorized as either primary (non-rechargeable) or secondary (rechargeable). In either case, a fully charged battery delivers electrical power as it undergoes an oxidation/reduction process and electrons are allowed to flow between the negative and positive polls of the battery.
Lithium ion batteries are secondary batteries; which means that they can be recharged by driving current in the opposite direction and reducing lithium ions to Li0 at the anode. A generalized schematic representation of a lithium ion battery is shown in FIG. 1. The direction of movement of ions and electrons are shown to represent “charging”. The discharge cycle would show ions moving in the opposite direction. Lithium ions migrating into the anode are met by electrons moving toward the anode through the closed circuit, thus reducing the Li+ to Li0 (lithium metal). Li0 is actually much larger in diameter than Li+ because of the electron it acquired occupies its 2S orbital. Consequently lithium metal occupies a significant amount of space. Conventional carbon anodes accommodate reduced lithium between the layers of graphite. Graphite can be thought of as 2-D arrays of 6-membered rings of carbon forming “sheets” that slide easily on one another. Fully charged, a graphite anode is able to accommodate the volume of lithium without imposing special demands beyond the inherent space that already exists between the sheets of graphite sheets.
There are no such special demands on the cathode as Li+ requires very little space (like adding sand to a bucket of gravel). The metal oxides and/or phosphates comprising the cathode stay in place. But only a finite number of Li+ ions (usually one or two) can pair with each cluster of metal oxides. Thus much greater space requirement of the cathode limits the specific charge capacity (charge per gram or charge per cubic millimeter). From the standpoint of size and molecular mass alone, lithium is the ideal element to use in batteries that must be made compact and light. In addition, lithium has the highest redox potential difference of any element.
The average cathode composition generally has lower charge capacity and therefore requires more size (and weight) when matched with the most common anode composite, graphite. As a consequence, a majority of research has focused on developing improved cathodes.
Some researchers have sought to develop alternative anodes for lithium ion batteries using silicon based materials. Silicon (Si) is known to have a far superior capacity to attract lithium than carbon used in traditional batteries [372 milliamp hours per gram, (mAh/g) versus 4,212 mAh/g for Si]. However, no commercial batteries have been successfully introduced using Si because no suitable structure has been found that prevents mechanical breakdown of the Si composites after only a few recharge cycles. Specifically, the limited structural form of silicon, coupled with the strong attraction that lithium has for silicon, results in mechanical failure due to volumetric expansion after a few charge/recharge cycles.
Accordingly, there is a need for new materials and methods that improve upon existing battery technology. In particular, there is a need for materials that provide a suitable porous framework to accommodate the spatial requirements of lithium accumulation at the anode of lithium ion batteries, and also possess good charge carrier mobility.
Also needed are nanoparticle materials that can be efficiently and economically produced from abundant and readily available raw materials. While particle size control has been demonstrated using such methodologies as plasma enhanced chemical vapor deposition (PECVD), hot wire chemical vapor deposition (HWCVD) and ion beam deposition (IBD), commercial production using these methods usually involve in situ film manufacturing. Group IVA nanoparticle powders are only available commercially in very limited range of specifications and only with dielectric passivation. The products are expensive because their production requires large capital costs for production equipment and high energy costs in production.